The present invention relates to a planar semiconductor device, and, more particularly, to an improvement in a field plate electrode for increasing the junction breakdown voltage of a planar junction.
A planar semiconductor device comprises an impurity diffusion region formed by diffusing an impurity into a semiconductor layer of one conductivity type from its surface. The impurity diffusion region has a conductivity type opposite to that of the semiconductor layer. A planar junction formed between the semiconductor layer and the impurity diffusion region has an arcuated cross-section.
FIG. 1 shows a cross-section of a semiconductor structure with a planar junction. Referring to FIG. 1, impurity diffusion region 2 of P.sup.+ conductivity type is provided in silicon layer 1 of N conductivity type. Region 2 is formed by diffusing an impurity into silicon layer 1. A planar PN junction is defined between layer 1 and region 2. Silicon oxide film 3 is formed on the major surface of layer 1. When the planar junction is reverse-biased, a depletion layer is formed at the planar junction. In FIG. 1, part of the depletion layer formed only on the side of layer 1 is shown by the broken line. Generally, most part of a depletion layer is formed in a semiconductor region of a low impurity concentration, and less part is formed in a semiconductor region of a high impurity concentration. The less part of a depletion layer is very small and, therefore, can be considered to be negligible.
A disadvantage of a planar junction is that it has a low breakdown voltage. This is mainly because, when the depletion layer is viewed in cross-section, the arcuated portion is relatively sharp and the portion of the depletion layer exposed on the surface of layer 1 is relatively narrow. When the depletion layer has such an arcuated portion the electric field is concentrated at the arcuated portion, causing breakdown. Further, when the portion of the depletion layer exposed on the surface of layer 1 is narrow, an electric field is concentrated at that portion of the depletion layer, causing breakdown.
A semiconductor device with a field plate electrode, as shown in FIG. 2, has been proposed. The semiconductor device increases the area of the depletion layer exposed on the semiconductor layer 1. In the device shown in FIG. 2, field plate electrode 4 of a metal, such as aluminum, is formed on film 3. Electrode 4 extends, when viewed through film 3, from the vicinity of the junction between layer 1 and region 2 to layer 1. A negative voltage is applied to electrode 4. Electrons existing in a surface region of layer 1 are discharged into the inner region. As shown in FIG. 2, the portion of the depletion layer exposed on the surface of layer 1 is increased, thereby improving the breakdown voltage. However, the depletion layer still has sharply curved portions. Because of this, the electric field is concentrated at the curved portion and a high breakdown voltage cannot be obtained.
In order to solve this problem, a further semiconductor device has been proposed, as illustrated in FIG. 3. A high-resistance polysilicon layer is used as field plate electrode 4A. One end Y (i.e., an end away from region 2) of high-resistance polysilicon layer 4A is connected to a positive potential, and the other end X (i.e., an end near region 2) is connected to a negative potential to cause small electric current "i" to flow from end Y to end X. Electric current "i" causes a voltage drop with a moderate potential gradient, as shown in FIG. 4. For this reason, the depletion layer shows a moderate gradient and has no sharply curved portion. Thus, electric field concentration can be eliminated, and a breakdown voltage can be increased, preventing breakdown. However, flow of electric current "i" results in a considerable power consumption. Furthermore, when the above structure is applied to a transistor, the transistor will fail to operate in a small-current region.